1. Field of the Invention
The invention relates to a spark plug for an internal combustion engine which is capable of reducing a voltage required to induce spark discharges while reducing carbon-related deposit even when using a shorter insulator nose.
2. Description of Prior Art
In a high compression type engine and a lean burn type engine each recently put into a practical use, the ignitable conditions come to worsen so that remnant of carbon, oil or the like (unburned material) tends to deposit on a front end surface of the insulator so as to induce carbon smoldering in the spark plug. In order to alleviate the carbon smoldering, it has been demanded to effectively remove the unburned material deposited on the front end surface of the insulator to impart a fouling resistant property.
In a spark plug disclosed by a laying-open application No. 2-181383, the carbon deposit on the insulator is burningly removed by subjecting the carbon deposit to inductive spark discharges. That is to say, it is possible to burningly remove the carbon deposit which is exposed to an ionization area of the inductive component among the spark discharges established across the center and ground electrode.
In a spark plug disclosed by U.S. Pat. Nos. 4,845,400 and 5,159,232, the carbon deposit on the insulator is burningly removed by subjecting the carbon deposit to inductive spark discharges in the same manner as described in the laying-open application No. 2-181383.
However, in the laying-open application No. 2-181383, it is necessary to extend the front end of the center electrode by 1.1 mm or less beyond that of the insulator when considering a relationship between a diametrical dimension of the front end of the center electrode and an inner diameter of the axial bore. Furthermore, the front end of the center electrode is likely to be excessively heated for the lack of its heat-drawing effect when heated by the spark discharge energy.
In the reference of U.S. Pat. No. 4,845,400, it is necessary to extend the front end of the center electrode by 1.0 mm or less beyond that of the insulator when considering a relationship between a diametrical dimension of the front end of the center electrode and an inner diameter of the axial bore. Furthermore, the front end of the center electrode is likely to be excessively heated for the lack of its heat-drawing effect when heated by the spark discharge energy.
In the reference of U.S. Pat. No. 5,159,232, it is necessary to define a diameter (d) of the front end of the center electrode as 0.6 mm.ltoreq.d.ltoreq.1.55 mm when considering a space between the front end of the center electrode and a front open end of the axial bore of the insulator. Furthermore, it is necessary to define how long the front end of the center electrode should extend beyond the insulator.
Therefore, it is a main object of the invention to provide a spark plug which is capable of preventing the carbon smoldering while improving the ignitability with a low voltage required to induce the spark discharges even when an insulator has a shorter insulator nose. This is realized by combining a very thinned front end of the center electrode and a space (referred often to as "air-pocket" hereinafter) between an outer surface of the center electrode and the inner wall of the front open end of the axial bore of the insulator.